Many uses of glass, including glass for producing LCDs, require a very clean glass surface that is substantially free of dust and other organic and/or inorganic contaminants. When exposed to the environment, glass surface can quickly become contaminated with dust and other inorganic and/or organic ambient contaminants, with contamination being observed within a few hours.
Current procedures used to cut and grind glass surfaces and edges often generate small glass chips. Such chips can have a size in the range between about 1 and 100 microns. Some of these particles irreversibly adhere to the clean glass surface, rendering the glass useless for many applications. This is particularly a serious problem for LCD glass surfaces.
LCD glass can be made by fusion draw process, which yields flat and smooth glass surfaces. The glass sheets can be cut to the desired size and then the edges ground. If water is actively involved between the surface and the glass chips generated during cutting and grinding, permanent chemical bonding may occur, rendering the adhesion of the glass chips to the surface irreversible.
One known method of protecting glass surfaces, specifically, surfaces of LCD glass sheets, is to apply a preformed polymer film on both major surfaces of the glass to protect the glass during the scoring, breaking and beveling process. In a typical method, one major surface has a polymer film attached with an adhesive, and the other major surface has a film attached by static charge. The first film is removed after the edge finishing (cutting and/or grinding) of the sheet is completed, while the second is removed prior to the finishing process. Although the adhesive-backed film protects the surface from scratching by the handling equipment, it causes other problems. Another problem with this film is that it may leave an adhesive residue on the glass surface. A further problem with the film approach is glass breakage during peeling of the film from the glass surface, especially for large and/or thin glass sheets.
Many polymer coatings, such as polyvinyl alcohol, can offer particle protection and scratch resistance capabilities. However, few of them can be completely removed in a cleaning solution at a temperature as low as 40° C. in a typical manufacturing process. One method of temporarily protecting glass surface, especially LCD glass surface, involves applying an aqueous solution of polysaccharides (e.g., a starch) to the glass surface, forming a protective coating of the polysaccharides on the glass surface by removing water from the solution, and then subsequently removing the polysaccharide-containing coating from the surface using an aqueous solution when desired to reveal the protected surface. The removing aqueous solution may contain a detergent. The polysaccharide coating formed on the glass surface offers particle protection and scratch resistance capabilities. However, the high water solubility of polysaccharides, especially starches, constitutes a potential drawback of this method. Before the cleaning step, glass sheets are usually subjected to other finishing steps such as cutting and edge grinding, in which water may be used as a cooling agent. Due to their high solubility in water, the polysaccharide coatings may be diminished during such stages, leading to reduced particle protection and scratch resistance.
A desirable property of the temporary protective coating for surface of LCD glass is its removability. Manufacturers of LCDs use the glass as the starting point for complex manufacturing processes in which semiconductors, e.g., thin film transistors, are formed on the glass substrates. In order not to adversely affect such processes, any protective coatings on the glass surface must be readily removable prior to the beginning of the LCD production process, without substantially changing the chemical and physical characteristics of the glass surface.
Therefore, there remains a need for an improved method of temporarily protecting the surface of glass using a coating system, especially glass for producing LCDs, from being contaminated by ambient contaminants, contaminants produced during the processing of the glass and/or scratching. The coating system should be easy to be removed and does not leave residue on the glass surface upon removal, whereby a substantially clean and coating-free surface can be restored for further use of the glass, e.g., for the production of LCDs.
In view of the foregoing, there has been a need in the art for a method for protecting surface of glass, especially glass sheets for the production of liquid crystal displays, which has the following characteristics:
(1) The method should be preferably one that can be easily incorporated in the overall glass forming process, specifically, at the end of the forming process, so that newly formed glass is protected substantially immediately after it is produced. Thus, the coating material should be able to withstand the environment of the glass forming line (e.g., high temperatures). In addition, the method should be safe to use in such an environment;
(2) The coating must offer sufficient protection to the glass surface from being adhered to and contaminated by contaminants produced during the processing of the glass sheet, including cutting and/or grinding, and/or ambient contaminants, organic and/or inorganic, that the glass surface typically may come into contact with during packing, storage and shipment prior to use;
(3) The coating must be sufficiently robust to continue to provide protection after being exposed to the substantial amount of water which typically comes into contact with the glass surface during the processing of the glass, including cutting and/or grinding. This requires that the coating system has a sufficiently low solubility in water under the processing condition;
(4) The coating should preferably protect the glass sheet from scratching during processing, handling, shipping, and storage (as used herein, scratching includes abrasion). More preferably, the coating should permit the glass sheets to be stacked very closely with minimal spacing materials between them during handling, shipping and storage;
(5) The coating should be substantially completely removable from the glass prior to its ultimate use in, for example, producing a liquid crystal display. Preferably, the removing condition should be mild and environmentally friendly; and
(6) The coating should preserve the pristine glass surface without substantially changing the surface's chemical composition and physical properties, e.g., smoothness, as a result of the coating process, the presence of coating on the surface during handling, shipping, storage and the subsequent removal of the coating from the surface.
The present invention addresses and satisfies this long-standing need in the art.